1. Field of the Invention
This invention relates to a filler cap structure in which a filler cap is adapted to be fitted to a filler opening or filler tube of a fuel tank for a vehicle such as an automobile.
2. Description of the Prior Art
In general, a vehicle adapted to be driven by an internal combustion engine has a fuel tank for supplying fuel to the internal combustion engine, and when the fuel stored in the fuel tank is reduced below a certain level, a filler cap fitted to a filler opening of the fuel tank is removed so that a filler nozzle leading to a source of fuel is inserted into the filler opening for filling the fuel tank with fuel. Further, when the fuel stored in the fuel tank is consumed below a certain level, the space above the fuel in the fuel tank is filled with fuel vapor so that the fuel vapor thus accumulated is discharged, upon removal of the filler cap, to the ambient atmosphere so as to cause air pollution.
Japanese Utility Model Laid Open No. 92653/82 discloses a filler cap structure for an automotive fuel tank in which when the pressure in the fuel tank is reduced abnormally, atmospheric air is introduced through a vent hole in the filler cap into the fuel tank so as to raise the pressure therein until the pressure in the fuel tank becomes equal to the atmospheric pressure, while on the other hand, upon a pressure rise in the fuel tank as a result of an increase in temperature in the ambient air, the fuel vapor in the fuel tank is discharged through a vent valve to the atmosphere. However, such a discharge of fuel vapor to the atmosphere is a major cause of air pollution.
Japanese Utility Model Laid Open No. 103746/'83 teaches a filler cap structure for an automotive fuel tank which is provided with a pressure control valve for controlling the pressure in the fuel tank substantially at a constant level. In this case, however, air pollution cannot be avoided since pressure variations in the fuel tank resulting from fuel vaporization or consumption of the fuel stored therein are controlled by the discharge of the fuel vapor to the atmosphere or the introduction of atmospheric air into the fuel tank.